To determine the concentration of parathyroid hormone related protein in amniotic fluid during the course of gestation. The production of PTHrP by the fetal lung is at first widespread. As the air sacs develop the production of PTHrP becomes restricted to the lining of the air sacs. During gestation the developing lungs produce fetal lung fluid which is secreted into the amniotic fluid. The path of the secreted fetal lung fluid follows that of exhaled air, through the nose and mouth into the amniotic cavity. We hypothesize that the lung contributes PTHrP to the amniotic fluid via fetal lung fluid. Further, we hypothesize that the concentrations of PTHrP increase during gestation similar to saturated phosphatidylcholine and phosphatidylglycerol.